The subject matter disclosed herein relates to an electrochemical system for separating hydrogen from contaminants in a gas stream substantially composed of hydrogen, and in particular to a system for separating hydrogen from a mixture containing carbon monoxide (“CO”) contaminant.
Polymer electrode membrane (“PEM”) systems use an electrochemical process to separate hydrogen from other elements or compounds. One example of PEM system is an electrolysis cell stack that separates water into hydrogen and oxygen gas. Another type of PEM system may be used to separate hydrogen from a gas stream comprised of hydrogen and other compounds such as CO2, CO, N2, etc. These types of PEM systems may be applicable to applications that use hydrogen as a process fluid, such as a heat treating operation where the heat treating is performed in a hydrogen atmosphere.
The PEM system is typically comprised of a plurality of cells arranged electrically in series. Each of the cells consists of a polymer membrane with an anode and a cathode electrode arranged on opposite sides. When the PEM system is used to process the exhaust gas stream in an application such the aforementioned heat treating operation, operation may be stalled when an appreciable concentrations carbon monoxide (“CO”) is present in the hydrogen is fed to an electrode in a PEM cell. The hydrogen is adsorbed on the catalyst and effectively stops the operation of the cell. When carbon monoxide (“CO”) is fed to the anode of an electrochemical hydrogen pump or compressor, thermodynamics predicts that a fraction of the CO will participate in an equilibrium reaction with water, which is available due to PEM hydration, and form CO2. Typically, in practice, this reaction is very slow and the electrode catalyst is rapidly de-activated by the CO.
Some prior art systems have tried to address this issue by operating at very high temperatures to accelerate the oxidation of CO. This often requires alternative, high temperature membranes. Another approach that is often explored is raising the cell voltage. When the cell voltage is raised, oxidation of CO is driven at the anode by the external power source, consuming water and forming hydrogen (“H2”) at the cathode. This approach works to a degree. Typically, the high voltage is applied momentarily in a pulse or wave function of some sort. However, anytime the voltage applied to the cell is elevated, the power consumption is elevated.
Pulsing the voltage to an elevated voltage also creates other problems. As the voltage is driven higher in an electrochemical hydrogen compressor, although oxidation of CO at the anode is promoted, more hydrogen is oxidized. In other words, the pump rate is increased. A portion of the resultant electrical current will go to oxidize hydrogen while another portion will go to oxidization of CO. So if a hydrogen pump or compressor is operating and the voltage is pulsed higher, the presence of the hydrogen on the anode functions almost as a sacrificial material, preventing the oxidation of CO in favor of oxidation of hydrogen.
Accordingly, while existing PEM systems are suitable for their intended purposes, the need for improvement remains, particularly in providing a PEM system where hydrogen is separated from CO at lower power consumption levels and/or lower operational temperatures.